net: app: Do not reset net_context information

Do not reset net_context information in net_app_close. In case of
TCP behind the curtains connection might wait for some timers to
expire and send some messages (e.g. ACK). If we set source port to
'0', unexpected behaviour might happen with peer connection.
Only reset net_app context related information on net_app_close.
Let net_context_put will take care of proper connection closure.

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>